Semiconductor device and method of manufacture

ABSTRACT

An image sensor includes a photosensitive sensor, a floating diffusion node, a reset transistor, and a source follower transistor. The reset transistor comprises a first source/drain coupled to the floating diffusion node and a second source/drain coupled to a first voltage source. The source follower transistor comprises a gate coupled to the floating diffusion node and a first source/drain coupled to the second source/drain of the reset transistor. A first elongated contact contacts the second source/drain of the reset transistor and the first source/drain of the source follower transistor. The first elongated contact has a first dimension in a horizontal cross-section and a second dimension in the horizontal cross-section. The second dimension is perpendicular to the first dimension, and the second dimension is less than the first dimension.

RELATED APPLICATION

This application is a divisional of U.S. Non-Provisional applicationSer. No. 17/520,930, titled “SEMICONDUCTOR DEVICE AND METHOD OFMANUFACTURE” and filed on Nov. 8, 2021, which is a divisional of U.S.Non-Provisional application Ser. No. 16/429,403, titled “SEMICONDUCTORDEVICE AND METHOD OF MANUFACTURE” and filed on Jun. 3, 2019, whichclaims priority to U.S. Provisional Application Ser. No. 62/773,289,titled “SEMICONDUCTOR ARRANGEMENT AND METHOD OF MANUFACTURE” and filedon Nov. 30, 2018. U.S. Non-Provisional application Ser. No. 17/520,930,U.S. Non-Provisional application Ser. No. 16/429,403, and U.S.Provisional Application Ser. No. 62/773,289 are incorporated herein byreference.

BACKGROUND

Image sensors are found in a variety of consumer electronics andindustrial electronics. For example, digital cameras, digital videosystems, and other image capture devices, such as copiers, scanners,etc., use image sensors to capture a scene and convert the scene into animage. One type of image sensor that is commonly used in image capturedevices is a solid-state image sensor, such as a complementarymetal-oxide semiconductor (CMOS) image sensor. When a pixel array of thesolid-state image sensor is exposed to light, photosensitive sensors ofthe pixel array convert the light into voltage. The voltage generated byrespective photosensitive sensors is subsequently measured and used togenerate or reconstruct an image.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a component block diagram illustrating an examplelight-capturing device, in accordance with some embodiments.

FIG. 2 is a component block diagram illustrating an example imagesensor, in accordance with some embodiments.

FIG. 3 is a schematic diagram illustrating a cluster of pixels and areadout circuit for the cluster of pixels, in accordance with someembodiments.

FIG. 4 is a plan view of a semiconductor device comprising a cluster ofpixels and a readout circuit for the cluster of pixels, in accordancewith some embodiments.

FIGS. 5-9 are illustrations of a semiconductor device at various stagesof fabrication, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended limiting. For example, the formation of afirst feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “over”, “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

A solid-state image sensor, such as a CMOS image sensor, typicallycomprises a plurality of pixels arranged to form a pixel array.Respective pixels comprise a photosensitive sensor, such as aphototransistor, a photodiode, or the like. Under normal operation,respective pixels switch between an integration period and an idleperiod. While a pixel is in an integration period, a series of processesare performed on the pixel to measure the electrical charge thataccumulates during the integration period. After the pixel has performedthe series of processes, and the accumulated electrical charge ismeasured, the pixel enters an idle period. While light is detected andconverted to electrical charge during the idle period, electrical chargegenerated while the pixel is in the idle period is generally notmeasured or used to generate an image.

FIG. 1 is a component block diagram illustrating an examplelight-capturing device 100, in accordance with some embodiments. In someembodiments, the light-capturing device 100 is configured to convertlight 105 that impinges the light-capturing device 100 into electricalsignals that are representative of the light 105. For example, in someembodiments, the voltage of an electrical signal generated by an imagesensor 110 of the light-capturing device 100 is proportional to anamount of light 105 that is detected by the image sensor 110. In someembodiments, the image sensor 110 comprises a back-illuminated imagesensor (BIS). Example light-capturing devices 100 include, among otherthings, digital cameras, digital video systems, scanners, copiers, andother devices that digitally generate an image of a scene. In someembodiments, the light-capturing device 100 includes devices that areconfigured to measure an amount of light over a specified exposure time,but not necessarily configured to generate an image based upon themeasured amount of light 105. As used herein, the term light 105 refersto electromagnetic radiation within the ultraviolet spectrum, visiblespectrum, and infrared spectrum, which generally have wavelengthsbetween about 10 nanometers and about 1 millimeter.

In some embodiments, the light-capturing device 100 comprises a lens115, a diaphragm 120, the image sensor 110, an image processor 125, anda display 130. In some embodiments, the lens 115 is substantiallytransparent to the light 105 and is configured to protect the imagesensor 110 or other sensitive electronics of the light-capturing device100, such as from dust, debris, fingerprints, etc. In some embodiments,the lens 115 is shaped to refract the light 105, causing the light 105to converge or to diverge, for example.

In some embodiments, the diaphragm 120 is configured to control exposureof the image sensor 110 to light 105. That is, in some embodiments, thediaphragm 120 regulates the amount of light 105 that passes from thelens 115 to the image sensor 110. As an example, in some embodiments,the diaphragm 120 is constructed of a plurality of adjustable finsshaped to form an aperture at approximately a center of the diaphragm120. Based upon a desired exposure, one or more of the fins are adjustedto increase the aperture, allowing more light 105 to impinge the imagesensor 110, for example, or to decrease the aperture, allowing lesslight 105 to impinge the image sensor 110, for example. In someembodiments, the diaphragm 120 is configured to adjust one or more finssuch that the aperture is removed or fully closed when it is desirableto shield the image sensor 110 from light 105.

In some embodiments, the image sensor 110 is configured to detect light105 impinging upon a detection surface of the image sensor 110 and toconvert the light 105 into electrical signals. For example, as furtherdescribed with respect to FIG. 2 , in some embodiments the image sensor110 comprises a pixel array. In some embodiments, respective pixels ofthe pixel array comprise a photosensitive sensor, such as a photodiodeor a phototransistor, configured to convert the light 105 intoelectrical charge. In some embodiments, the electrical charge generatedduring an integration period is read out of the pixel and measured todetermine an amount of light that impinged the photosensitive sensorduring the integration period. In some embodiments, the pixel array ofthe image sensor 110 is a solid-state pixel array. In some embodiments,the pixel array of the image sensor 110 is a complementary metal-oxidesemiconductor array (CMOS array).

In some embodiments, the image processor 125 is configured to collectmeasurements or analog signals yielded from respective pixels of thepixel array and to generate or reconstruct an image based upon themeasurements. In some embodiments, the image processor 125 is configuredto convert analog signals output by the image sensor 110 into digitalsignals, which are accumulated and stored in a memory.

In some embodiments, the display 130 is configured to display an imagegenerated or reconstructed by the image processor 125 from the detectedlight 105. Accordingly, the display 130 presents, such as to a user, arendering of a scene that the light-capturing device 100 is focusedupon.

FIG. 2 is a component block diagram illustrating an example image sensor110 configured to detect light 105 and to convert the light 105 intoelectrical charge, in accordance with some embodiments.

In some embodiments, the image sensor 110 comprises a control circuit200, a pixel array 205, and a readout component 210. In someembodiments, the control circuit 200 is configured to drive pixels 215of the pixel array 205 via control signals that are transmitted to thepixels 215. In some embodiments, multiple pixels 215 are coupled to asame control channel through which the control signals are transmittedbetween the control circuit 200 and the pixels 215. Therefore, the samecontrol signal is applied to multiple pixels 215. For example, in theillustrated embodiment, the pixels 215 are arranged in clusters 220,with each cluster comprising four pixels 215. In the illustratedembodiment, one pixel 215 from the upper left cluster 220 and one pixel215 from the upper right cluster 220 are coupled to the same controlchannel and therefore receive the same control signal.

It is to be appreciated that the illustrated embodiment is merely oneexample arrangement for clustering pixels and that other arrangementsare contemplated. For example, in some embodiments, the pixels arearranged in rows and columns, and the control circuit 200 is configuredfor row-by-row control. That is, in some embodiments, control signalstransmitted via a first control channel are configured to drive pixels215 of a first row, control signals transmitted via a second controlchannel are configured to drive pixels 215 of a second row, etc. Thus,according to some embodiments, pixels 215 of the first row share acommon control channel, pixels 215 of the second row share commoncontrol channel, etc. In some embodiments, the control circuit 200 isconfigured for pixel-by-pixel control, such that the control circuit 200is configured to control each pixel 215 separately. Moreover, while theillustrated embodiment has four pixels 215 per cluster, any number ofpixels 215 may be arranged to form a cluster.

In some embodiments, the pixel array 205 is configured to detect lightimpinging a detection surface of the pixel array 205 and to convert thelight into electrical charge. For example, as further described withrespect to FIG. 3 , in some embodiments, respective pixels 215 of thepixel array 205 comprise a photosensitive sensor configured to convertlight into electrical charge. In some embodiments, charge thataccumulates at the photosensitive sensor while the pixel 215 is in anintegration period, defined based upon a desired exposure time, forexample, is utilized to generate a readout signal that is transmittedover a readout channel to the readout component 210.

In the illustrated embodiment, pixels 215 arranged in a same cluster 220share a common readout channel through which signals are transmittedfrom the pixels 215 within the cluster 220 to the readout component 210.The readout component 210 is configured to read out respective pixels215 to determine an amount of charge measured by respective pixels 215,for example. In some embodiments, the readout component 210 isconfigured to perform a correlated double sampling (CDS) process, usingthe readout signals from respective pixels 215, to determine an amountof charge measured by respective pixels 215. In some embodiments, thereadout component 210 is configured to use other readout processes inaddition to a CDS process or as substitution for a CDS process todetermine an amount of charge measured by respective pixels 215.

In some embodiments, fewer than all pixels 215 within a cluster 220share a common readout channel. Moreover, other configurations arecontemplated. For example, in some embodiments wherein the pixels arearranged in rows and columns, and the readout component 210 isconfigured to read out the pixels 215 column-by-column. That is, readoutsignals generated by pixels 215 of a first column are transmitted over afirst readout channel, readout signals generated by pixels 215 of asecond column are transmitted over a second readout channel, etc. Insome embodiments, the pixels 215 are read out pixel-by-pixel, where notwo pixels share a readout channel.

FIG. 3 is a schematic diagram illustrating a cluster 220 of pixels 215of the pixel array 205 and an example readout circuit 300, in thereadout component 210, to which the cluster of pixels 215 are coupled,in accordance with some embodiments. In some embodiments, the readoutcomponent 210 comprises multiple instances of the readout circuit 300.For example, if the pixel array 205 comprises N pixels 215, and eachreadout circuit 300 is shared by M pixels 215, the readout component 210comprises N/M instances of the readout circuit 300. In some embodiments,M is equal to the number of clusters 220. For example, returning to FIG.2 , which illustrates pixels 215 of a same cluster 220 sharing a singlereadout channel, the number of readout circuits 300 may be equal to thenumber of clusters 220.

In some embodiments, each pixel 215 comprises a transfer transistor 310and a photosensitive sensor 315. Each pixel 215 is configured to measureor sample light to which the image sensor 110 is exposed. Moreparticularly, each pixel 215 is configured to convert light impingingthe pixel 215 into electrical charge that is measured to approximate anamount of light that impinged the pixel 215 during an integrationperiod, or a sampling period, for example. In some embodiments, thephotosensitive sensor 315 comprises a photodiode. In some embodiments,the photosensitive sensor 315 comprises a phototransistor.

In some embodiments, the readout circuit 300 comprises transistors. Thetransistors have source/drains. Generally, depending on how thetransistor is oriented in a circuit and the direction current flow, oneof the source/drains operates as a drain and the other source/drainoperates as a source. The term, “source/drain” refers generically to oneof the source/drains that may operate as a source or as a drain of thetransistor. Moreover, while specific reference is made below to a drainand a source of a transistor, in some embodiments, the location of thedrain and the source is reversed. That is, what is referenced below as adrain of a transistor may, in some embodiments, instead be the source ofthe transistor and what is referenced below as a source of thetransistor may, in some embodiments, instead be the drain of thetransistor.

In some embodiments, the readout circuit 300 comprises a reset (RES)transistor 320, a source follower (SF) transistor 325, and a row select(RS) transistor 330. In some embodiments, one or more of the transfertransistors 310, the reset transistor 320, the source followertransistor 325, or the row select transistor 330 comprises an n-typemetal oxide semiconductor (NMOS) transistor. In some embodiments, when alogic high voltage is applied to a gate of an NMOS transistor, the NMOStransistor is turned on or enabled, forming a connection from a sourceof the NMOS transistor to a drain of the NMOS transistor. In someembodiments, one or more of the transfer transistors 310, the resettransistor 320, the source follower transistor 325, or the row selecttransistor 330 comprises a p-type metal oxide semiconductor (PMOS)transistor. In some embodiments, when a logic low voltage, such asground, is applied to a gate of a PMOS transistor, the PMOS transistoris turned on or enabled, forming a connection from a source of the PMOStransistor to a drain of the PMOS transistor. As used herein, enabling atransistor comprises applying an appropriate voltage to a gate of thetransistor to create a conductive channel between a source and a drainof the transistor.

In some embodiments, a drain 320D of the reset transistor 320 is coupledto a first voltage source 335, VDD. In some embodiments, drains 310D ofthe transfer transistors 310 are coupled to a source 320S of the resettransistor 320 at a floating diffusion (FD) node 340. In someembodiments, sources 3105 of the transfer transistors 310 are coupled tofirst terminals 315A of the associated photosensitive sensors 315. Insome embodiments, second terminals 315B of the photosensitive sensors315 are coupled to a second voltage source 345, Vss. In someembodiments, the second voltage source 345 comprises ground. In someembodiments, a drain 325D of the source follower transistor 325 iscoupled to the first voltage source 335. In some embodiments, a source325S of the source follower transistor 325 is coupled to a drain 330D ofthe row select transistor 330. In some embodiments, a gate 325G of thesource follower transistor 325 is coupled to the FD node 340. In someembodiments, an output signal, VOuT, for the readout circuit 300 isgenerated at an output terminal 350 coupled to a source 330S of the rowselect transistor 330.

In some embodiments, the gates 310G of the transfer transistors, thegate 320G of the reset transistor 320, and the gate 330G of the rowselect transistor 330 are coupled to the control circuit 200 of FIG. 2to receive control signals to enable the readout operation and to selectthe pixel 215 being read. In some embodiments, the control circuit 200resets the FD node 340 by enabling the reset transistor 320 to couplethe FD node 340 to the first voltage source 335.

In some embodiments, the control circuit 200 reads a particular pixel215 by providing a logic signal to the gate 310G to enable the selectedtransfer transistor 310 and by providing a logic signal to the gate 330Gto enable the row select transistor 330. An output of a pixel 215, suchas a readout signal indicative of an amount of electrical chargemeasured during an integration period, for example, is output from thepixel 215 to the readout circuit 300 through the FD node 340. In someembodiments, enabling a selected transfer transistor 310 transferscharge accumulated on the associated photosensitive sensor 315 to the FDnode 340. In some embodiments, the transferred charge creates a voltageon the gate 325G of the source follower transistor 325, therebygenerating a voltage on the source 325S of the source followertransistor 325 dependent on the voltage at the FD node 340. In someembodiments, enabling the row select transistor 330 allows the voltageat the source 325S of the source follower transistor 325 to be read atthe output terminal 350.

As further described with respect to FIG. 4 , according to someembodiments, a contact 355 is provided for the source 320S of the resettransistor 320 to allow interconnection with the FD node 340. In someembodiments, a contact 360 is provided for the drain 320D of the resettransistor 320 and the drain 325D of the source follower transistor 325to allow interconnection with the first voltage source 335. In someembodiments, the drain 320D of the reset transistor 320 and the drain325D of the source follower transistor 325 comprise a shared drain. Insome embodiments, a contact 365 is provided for the source 330S of therow select transistor 330 to allow interconnection with the outputterminal 350. In some embodiments, at least one of the contacts 355,360, 365 comprises an elongated contact having a first dimension 370greater than a second dimension 375 in a horizontal cross-section.

FIG. 4 is a plan view of a semiconductor device 400 comprising a cluster220 of pixels 215 and the readout circuit 300, in accordance with someembodiments. In some embodiments, the reset transistor 320 is positionedadjacent the source follower transistor 325 in a device region 405 ofthe semiconductor device 400. In some embodiments, the row selecttransistor 330 is positioned adjacent the source follower transistor 325in the device region 405. In some embodiments, the source followertransistor 325 is positioned between the reset transistor 320 and therow select transistor 330. According to some embodiments, the pixels 215are positioned in an array region 410 of the semiconductor device 400.In some embodiments, the photosensitive sensors 315 are positioned in agrid arrangement with the FD node 340 positioned in a center portion ofthe grid. The gates 310G of the transfer transistors 310 are positionedin the array region 410 over the associated photosensitive sensors 315.In some embodiments, an isolation structure 420 is positioned adjacentthe source 320S of the reset transistor. In some embodiments, anisolation structure 425 is positioned adjacent the source 330S of therow select transistor 330.

According to some embodiments, a node contact 340C contacts the FD node340. In some embodiments, a supply contact 345C contacts thephotosensitive sensors 315 to allow interconnection with the secondvoltage source 345. In some embodiments, gate contacts 310C contact thegates 310G of the transfer transistors 310. In some embodiments, a gatecontact 320C contacts the gate 320G of the reset transistor 320. In someembodiments, a gate contact 325C contacts the gate 325G of the sourcefollower transistor 325. In some embodiments, a gate contact 330Ccontacts the gate 330G of the row select transistor 330. In someembodiments, the contact 355 is positioned over and contacts the source320S of the reset transistor 320. In some embodiments, the contact 360is positioned over and contacts the drain 320D of the reset transistor320 and the drain 325D of the source follower transistor 325. In someembodiments, the contact 365 is positioned over and contacts the source330S of the row select transistor 330. In some embodiments, the contacts310C, 320C, 325C, 330C are positioned in a dielectric layer positionedover the gates 310G, 320G, 325G, 330G. In some embodiments, the contacts340C, 345C, 355, 360, 365 are positioned in a dielectric layerpositioned at least one of over or adjacent to the gates 310G, 320G,325G, 330G.

According to some embodiments, at least one of the contacts 355, 360,365 comprises an elongated contact having a first dimension 370 greaterthan a second dimension 375 in a horizontal cross-section. As usedherein, a horizontal cross-section refers to a cross-section of thedevice at a plane that is parallel to a top surface of a semiconductorlayer 415, illustrated in FIG. 5 . Thus, according to some embodiments,at least one of the contacts 355, 360, 365 comprises an elongatedcontact having a first dimension greater than a second dimension in theplan view illustrated in FIG. 4 .

In some embodiments, a ratio between the first dimension and the seconddimension is between about 1.2 and 10. In some embodiments, at least oneof the gate contacts 310C, 320C, 325C, 330C has a second dimension in ahorizontal cross-section greater than the second dimension in the samedirection of at least one of the contacts 355, 360, 365. In someembodiments, at least one of the gate contacts 310C, 320C, 325C, 330Chas a first dimension in a horizontal cross-section less than a firstdimension in the same direction of at least one of the contacts 355,360, 365. In some embodiments, the horizontal cross-section areas of thecontacts 310C, 320C, 325C, 330C, 340C, 345C, 355, 360, 365 isapproximately the same, thereby providing an approximately equal contactvolume. However, the elongated cross-section of at least one of thecontacts 355, 360, 365 allows adjacent structures to be positioned moreclosely together, reducing a footprint of the readout circuit 300. Insome embodiments, where the contact 355 is elongated, the spacingbetween the isolation structure 420 and the gate 320G of the resettransistor 320 is reduced as compared to an implementation where thefirst dimension of the contact 355 is equal to the second dimension ofthe contact 355. In some embodiments, where the contact 360 iselongated, the spacing between the gate 320G and the gate 325G of thesource follower transistor 325 is reduced as compared to animplementation where the first dimension of the contact 360 is equal tothe second dimension of the contact 360. In some embodiments, where thecontact 365 is elongated, the spacing between the isolation structure425 and the gate 330G of the row select transistor 330 is reduced ascompared to an implementation where the first dimension of the contact365 is equal to the second dimension of the contact 365. In someembodiments, the elongated cross-section of at least one of the contacts355, 360, 365 allows the spacing to be reduced without increasingcontact resistance. In some embodiments, the elongated contact has arectangular horizontal cross-section shape. In some embodiments, theelongated contact has an elliptical horizontal cross-section shape. Insome embodiments, interconnections between the node contact 340C and thecontact 355, between the contact 360 and the first voltage source 335,and between the contact 365 and the output terminal 350 are positionedin a dielectric layer formed over the contacts 355, 360, 365.

FIGS. 5-9 are illustrations of the semiconductor device 400 at variousstages of fabrication, in accordance with some embodiments. FIGS. 5-9are cross-section views of the semiconductor device 400 taken throughthe device region 405 along line “X-X” in FIG. 4 . In some embodiments,the semiconductor device 400 comprises a plurality of layers formed overa semiconductor layer 415. In some embodiments, the semiconductor layer415 is part of a substrate comprising at least one of an epitaxiallayer, a single crystalline semiconductor material such as, but notlimited to Si, Ge, SiGe, InGaAs, GaAs, InSb, GaP, GaSb, InAlAs, GaSbP,GaAsSb, and InP, a silicon-on-insulator (SOI) structure, a wafer, or adie formed from a wafer.

According to some embodiments, the reset transistor 320, the sourcefollower transistor 325, and the row select transistor 330 are formed inand over the semiconductor layer 415 in the device region 405. In someembodiments, the gates 320G, 325G, 330G are formed over thesemiconductor layer 415. In some embodiments, the gates 320G, 325G, 330Gcomprise a gate insulation layer and a gate electrode.

According to some embodiments, the gates 320G, 325G, 330G are formed byforming a gate insulation layer, a layer sacrificial electrode materialover the gate insulation layer, and a hard mask layer over thesacrificial electrode material. In some embodiments, a patterningprocess is performed to pattern the hard mask layer corresponding to thepattern of gates 320G, 325G, 330G to be formed, and an etch process isperformed using the patterned hard mask layer to etch the sacrificialelectrode layer and the gate insulation layer to define the gates 320G,325G, 330G. In some embodiments, the sacrificial electrode materialcomprises polysilicon. In some embodiments, remaining portions of thehard mask layer form a cap layer 430 over the gates 320G, 325G, 330G. Insome embodiments, the gate insulation layer comprises silicon andoxygen. In some embodiments, the gate insulation layer comprises ahigh-k dielectric material. As used herein, the term “high-k dielectric”refers to the material having a dielectric constant, k, greater than orequal to about 3.9, which is the k value of SiO₂. The material of thehigh-k dielectric layer may be any suitable material. Examples of thematerial of the high-k dielectric layer include but are not limited toAl₂O₃, HfO₂, ZrO₂, La₂O₃, TiO₂, SrTiO₃, LaAlO₃, Y₂O₃, Al₂O_(x)N_(y),HfO_(x)N_(y), ZrO_(x)N_(y), La₂O_(x)N_(y), TiO_(x)N_(y), SrTiO_(x)N_(y),LaAlO_(x)N_(y), Y₂O_(x)N_(y), SiON, SiN_(x), a silicate thereof, and analloy thereof. Each value of x is independently from 0.5 to 3, and eachvalue of y is independently from 0 to 2.

In some embodiments, a replacement process is performed to remove thesacrificial material and the gate insulation layer and form replacementgates comprising a replacement gate insulation layer and a replacementgate electrode. In some embodiments, the replacement gate electrodescomprise metal fill material. In some embodiments, the replacement gateelectrodes comprise a conductive work function material layer formedover the gate insulation layer, and the metal fill material is formedover the work function material layer. In some embodiments, the workfunction material layer comprises a p-type work function material layer,such as at least one of TiN, TaN, Ru, Mo, Al, WN, ZrSi₂, MoSi₂, TaSi₂,NiSi₂, WN, or other suitable p-type work function materials. In someembodiments, the work function material layer comprises an n-type workfunction metal, such as at least one of Ti, Ag, TaAl, TaAlC, TiAlN, TaC,TaCN, TaSiN, Mn, Zr, or other suitable n-type work function materials.In some embodiments, the work function material layer comprises aplurality of layers. In some embodiments, the metal fill materialcomprises tungsten (W) or other suitable material. In some embodiments,the gate insulation layer and the material of the gate electrodes aredeposited by at least one of thermal oxidation, chemical oxidation,chemical vapor deposition (CVD), including low pressure CVD (LPCVD),plasma enhanced CVD (PECVD), ultra-high vacuum CVD (UHVCVD), reducedpressure CVD (RPCVD), atomic layer deposition (ALD), physical vapordeposition, pulsed laser deposition, sputtering, evaporative deposition,vapor phase epitaxy (VPE), molecular beam epitaxy (MBE), liquid phaseepitaxy (LPE), electroplating, electroless plating, or other applicabletechniques.

In some embodiments, sidewall spacers 435 are formed adjacent the gates320G, 325G, 330G. In some embodiments, the sidewall spacers 435 areformed by depositing a spacer layer over the gates 320G, 325G, 330G andperforming an anisotropic etch process to remove portions of the spacerlayer overlying horizontal surfaces of the cap layer 430 and thesemiconductor layer 415. In some embodiments, the sidewall spacers 435comprise the same material composition as the cap layer 430. In someembodiments, the sidewall spacers 435 comprise nitrogen and silicon. Insome embodiments, the sidewall spacers 435 comprise silicon and oxygen.In some embodiments, the sidewall spacers 435 comprise silicon andcarbon. In some embodiments, an implantation process is performed todefine the sources 320S, 325S, 330S and drains 320D, 325D, 330D in thesemiconductor layer 415 using the sidewall spacers 435 and the gates320G, 325G, 330G as an implantation mask.

In some embodiments, a dielectric layer 440 is formed between and overthe gates 320G, 325G, 330G. In some embodiments, a deposition process isperformed to form the dielectric layer 440. In some embodiments, thedielectric layer 440 comprises silicon dioxide or a low-k material. Insome embodiments, the dielectric layer 440 comprises one or more layersof low-k dielectric material. Low-k dielectric materials have a k-value(dielectric constant) lower than about 3.9. Some low-k dielectricmaterials have a k-value lower than about 3.5 and may have a k-valuelower than about 2.5. The materials for the dielectric layer 440comprise at least one of Si, O, C, or H, such as SiCOH and SiOC, orother suitable materials. Organic material such as polymers may be usedfor the dielectric layer 140. In some embodiments, the dielectric layer440 comprises one or more layers of a carbon-containing material,organo-silicate glass, a porogen-containing material, or combinationsthereof. The dielectric layer 440 may also comprise nitrogen in someembodiments. The dielectric layer 440 may be formed by using, forexample, at least one of PECVD, LPCVD, atomic layer CVD (ALCVD), or aspin-on technology. In some embodiments where the dielectric layer 440is formed using PECVD, the dielectric layer 440 is deposited at asubstrate temperature in a range of about 25° C. to about 400° C. and ata pressure of less than 100 Torr.

Referring to FIG. 6 , a patterned mask layer 445 is formed over thedielectric layer 440, in accordance with some embodiments. In someembodiments, contact openings 445A, 445B, 445C are defined in thepatterned mask layer 445. In some embodiments, the patterned mask layer445 comprises a plurality of individually formed layers that togetherdefine a mask stack. In some embodiments, the patterned mask layer 445comprises a hard mask layer formed over the dielectric layer 440 by atleast one of CVD, LPCVD, PECVD, UHVCVD, RPCVD, ALD, physical vapordeposition, pulsed laser deposition, sputtering, evaporative deposition,VPE, MBE, LPE, or other applicable techniques. In some embodiments, thehard mask layer comprises silicon and nitrogen. In some embodiments, thepatterned mask layer 445 comprises a bottom antireflective coating(BARC) layer formed over the hard mask layer. In some embodiments, theBARC layer is a polymer layer that is applied using a spin coatingprocess. In some embodiments, the patterned mask layer 445 comprises anorganic planarization layer (OPL) formed over the BARC layer. In someembodiments, the OPL layer comprises a photosensitive organic polymerthat is applied using a spin coating process. In some embodiments, theOPL layer comprises a dielectric layer. In some embodiments, thepatterned mask layer 445 comprises a photoresist layer formed over theOPL layer. In some embodiments, the photoresist layer is formed by atleast one of spinning, spray coating, or other applicable techniques. Insome embodiments, the photoresist layer comprises an electromagneticradiation sensitive material and properties, such as solubility, of thephotoresist layer are affected by electromagnetic radiation. Thephotoresist layer is either a negative photoresist or a positivephotoresist. In some embodiments, portions of the OPL layer that areirradiated by the electromagnetic radiation that patterns thephotoresist layer are affected to change the etch selectivity of theirradiated portions of the OPL layer with respect to non-irradiatedportions. Referring to FIG. 6 , the patterned mask layer 445 ispatterned by exposing the photoresist layer using a radiation source anda reticle to define a pattern in the photoresist layer. In someembodiments, exposed portions of the photoresist layer are removed todefine a patterned photoresist layer. In some embodiments, theunderlying OPL layer, BARC layer, and hard mask layer are etched usingthe patterned photoresist layer as a template to form the patterned masklayer 445 and expose portions of the dielectric layer through thecontact openings 445A, 445B, 445C.

Referring to FIG. 7 , portions of the dielectric layer 440 exposed byand underlying the contact openings 445A, 445B, 445C are removed todefine contact recesses 440A, 440B, 440C in the dielectric layer 440. Insome embodiments, an etch process is performed to remove the portions ofthe dielectric layer 440. In some embodiments, the contact recesses440A, 440B, 440C have an elongated horizontal cross-section shapecorresponding to the contacts 355, 360, 365 illustrated in FIG. 3 . Insome embodiments, the contact recesses 440A, 440B, 440C expose only aportion of the width 450 of the sidewall spacers 435. In someembodiments, the contact recesses 440A, 440B, 440C do not expose the caplayers 430.

Referring to FIG. 8 , portions of the sidewall spacers 435 exposed inthe contact recesses 440A, 440B, 440C are recessed, in accordance withsome embodiments. In some embodiments, an etch process is performed toremove the portions of the sidewall spacers 435. In some embodiments,the etch process is controlled such that the removed portions of thesidewall spacers 435 extend only a portion 435R of the height 435H ofthe sidewall spacers 435 taken from an upper surface of the sidewallspacers 435. In some embodiments, the process for etching the sidewallspacers 435 comprises a vertical etch and the duration of the verticaletch is selected such that merely a portion of the sidewall is removed.In some embodiments, the sidewall spacers 435 and the cap layers 430comprise different materials to allow selective etching of the sidewallspacers 435 while preserving the cap layer 430 in cases wheremisalignment may result in the cap layer 430 at least partially beingexposed through at least one of the openings 445A, 445B, 445C. In someembodiments, the portion 435R of the height comprises between 0% and 90%of the height 435H of the sidewall spacers 435. According to someembodiments, removing the portions of the sidewall spacers 435 increasesthe volume of the contact openings 445A, 445B, 445C. In someembodiments, the recessing the sidewall spacers 435 less than 90% avoidswidening the contact openings 445A, 445B, 445C at the interface with thesources 320S, 330S or drains 320D, 325D defined in the semiconductorlayer 415.

In some embodiments, the portions of the sidewall spacers 435 arerecessed concurrently with the removal of the portions of the dielectriclayer 440. In some embodiments, the material composition of thedielectric layer 440 is different than the material composition of thesidewall spacers 435. For example, the dielectric layer 440 may comprisesilicon oxide and the sidewall spacers 435 may comprise silicon nitride.In some embodiments, due to the difference in material compositions, thedielectric layer 440 is etched at a faster rate than the sidewallspacers 435. In some embodiments, due to this difference in etch rate,the sources 320S, 330S and drains 320D, 325D underlying the dielectriclayer 440 are exposed after only a portion of the sidewall spacers 435are recessed, resulting in removed portions of the sidewall spacers 435extending only a portion 435R of the height 435H of the sidewall spacers435. In some embodiments, the etch process ends upon the sources 320S,330S and drains 320D, 325D underlying the dielectric layer 440 beingexposed.

Referring to FIG. 9 , the patterned mask layer 445 is removed and thecontacts 355, 360, 365 are formed in the contact openings 445A, 445B,445C. In some embodiments, an etch process was performed to remove thepatterned mask layer 445. In some embodiments, the contacts 355, 360,365 comprise a metal. In some embodiments, the contacts 355, 360, 365comprise a metal silicide. In some embodiments, the contacts 355, 360,365 overlie the associated sidewall spacers 435 by an amount betweenabout 1-30% of the distance between the bottom edge of the sidewallspacer and the adjacent gate 320G, 325G, 330G. In some embodiments, theoverlie amount of less than 30% avoids increasing parasitic capacitancewith the gates 320G, 326G, 330G or causing a short to one of the gates320G, 326G, 330G due to overlay error. In some embodiments, the contacts355, 360, 365 are laterally spaced apart from the cap layer 430 by thesidewall spacers 435. In some embodiments, at least one of the contacts355, 360, 365 contacts the sidewall spacer 435 without contacting anupper surface of the cap layer 430. In some embodiments, a portion ofthe dielectric layer 440 separates an upper surface of the cap layer 430from a sidewall of the associated contact 355, 360, 365.

According to some embodiments, another patterned etch mask is formedhaving openings over the gates 320G, 325G, 330G and an etch process isperformed to form contact recesses for the contacts 310C, 320C, 325C,330C, 340C 345C shown in FIG. 4 . In some embodiments, the patternedmask layer 445 comprises openings for the contacts 355, 360, 365 and thecontacts 310C, 320C, 325C, 330C, 340C 345C.

According to some embodiments, the narrower second dimension 375 of theelongated contacts, such as at least one of the contacts 355, 360, 365allows the spacing between the reset transistor 320 and the sourcefollower transistor 325 to be reduced, reducing a footprint of thereadout circuit 300, as compared to an implementation where all of thecontacts 310C, 320C, 325C, 330C, 340C, 345C, 355, 360, 365 have the samesecond dimension 375. In some embodiments, the narrower second dimensionallows the spacing between the isolation structure 420 and source 320Sof the reset transistor 320 and the spacing between the isolationstructure 425 and the source 330S of the row select transistor 330 to bereduced, further reducing a footprint of the readout circuit 300.

According to some embodiments, an image sensor including aphotosensitive sensor, a floating diffusion node, a reset transistor,and a source follower transistor is provided. The reset transistorincludes a first source/drain coupled to the floating diffusion node anda second source/drain coupled to a first voltage source. The sourcefollower transistor includes a gate coupled to the floating diffusionnode and a first source/drain coupled to the second source/drain of thereset transistor. A first elongated contact contacts the secondsource/drain of the reset transistor and the first source/drain of thesource follower transistor. The first elongated contact has a firstdimension in a horizontal cross-section and a second dimension in thehorizontal cross-section. The second dimension is perpendicular to thefirst dimension, and the second dimension is less than the firstdimension.

According to some embodiments, the image sensor includes a transfertransistor including a first source/drain coupled to the photosensitivesensor and a second source/drain coupled to the floating diffusion node.

According to some embodiments, the image sensor includes a secondelongated contact contacting the first source/drain of the resettransistor and the floating diffusion node. The second elongated contacthas a third dimension in the horizontal cross-section and a fourthdimension in the horizontal cross-section. The third dimension isperpendicular to the fourth dimension, and the fourth dimension is lessthan the third dimension.

According to some embodiments, the image sensor includes a firstsidewall spacer adjacent a gate of the reset transistor and a secondsidewall spacer adjacent a gate of the source follower transistor. Thefirst elongated contact overlies at least one of the first sidewallspacer or the second sidewall spacer.

According to some embodiments, the first elongated contact contacts theat least one of the first sidewall spacer or the second sidewall spacer.

According to some embodiments, a semiconductor device including a firstgate, a first sidewall spacer adjacent the first gate, and a firstsource/drain is provided. The first sidewall spacer is between the firstgate and the first source/drain. The semiconductor device includes asecond gate, a second sidewall spacer between the first source/drain andthe second gate, and a first contact contacting the first source/drainand between the first sidewall spacer and the second sidewall spacer.The first contact contacts at least one of the first sidewall spacer orthe second sidewall spacer, and the first contact overlies the at leastone of the first sidewall spacer or the second sidewall spacer.

According to some embodiments, the first contact has a first dimensionin a horizontal cross-section and a second dimension in the horizontalcross-section, the first dimension is perpendicular to the seconddimension, and the second dimension is less than the first dimension.

According to some embodiments, the semiconductor device includes a firstgate contact contacting the first gate. The first gate contact has athird dimension greater than the second dimension of the first elongatedcontact, and the third dimension extends in a same direction as thesecond dimension.

According to some embodiments, the first gate contact has a fourthdimension less than the first dimension of the first elongated contact,and the fourth dimension extends in a same direction as the firstdimension.

According to some embodiments, the semiconductor device includes a caplayer positioned over a top surface of the first gate. The first contactis spaced apart from the cap layer by the first sidewall spacer.

According to some embodiments, the first contact contacts the firstsidewall spacer and the second sidewall spacer, and the first contactoverlies the first sidewall spacer and the second sidewall spacer.

According to some embodiments, the semiconductor device includes asecond source/drain, a third sidewall spacer adjacent the first gate andbetween the second source/drain and the first gate, an isolationstructure adjacent the second source/drain, and a second contactcontacting the first sidewall spacer. The second source/drain is betweenthe third sidewall spacer and the isolation structure. The secondcontact has a third dimension in a horizontal cross-section and a fourthdimension in the horizontal cross-section. The third dimension isperpendicular to the fourth dimension, and the fourth dimension is lessthan the third dimension,

According to some embodiments, the first contact has one of arectangular horizontal cross-section or an elliptical horizontalcross-section.

According to some embodiments, a method for forming a semiconductordevice includes forming a first gate. A first sidewall spacer is formedadjacent the first gate. A first source/drain is formed. A dielectriclayer is formed over the first gate, the first sidewall spacer, and thefirst source/drain. The dielectric layer is etched to define a contactrecess exposing the first sidewall spacer and the first source/drain.The first sidewall spacer exposed through the contact recess is etched.A first contact is formed in the contact recess contacting the firstsidewall spacer and the first source/drain.

According to some embodiments, the first contact overlies the firstsidewall spacer.

According to some embodiments, the first contact has a first dimensionin a horizontal cross-section and a second dimension in the horizontalcross-section. The first dimension is perpendicular to the seconddimension and the second dimension is less than the first dimension.

According to some embodiments, the method includes forming a secondsidewall spacer adjacent the first gate and forming a secondsource/drain, wherein the second sidewall spacer is between the firstgate and the second source/drain. An isolation structure is formedadjacent the second source/drain and a second contact contacting thesecond source/drain and the second spacer is formed. The second contacthas a third dimension in the horizontal cross-section and a fourthdimension in the horizontal cross-section. The third dimension isperpendicular to the fourth dimension, and the fourth dimension is lessthan the third dimension.

According to some embodiments, the method includes forming a first gatecontact contacting the first gate. The first gate contact has a thirddimension greater than the second dimension of the first contact, andthe third dimension extends in a same direction as the second dimension.The first gate contact has a fourth dimension less than the firstdimension of the first contact, and the fourth dimension extends in asame direction as the first dimension.

According to some embodiments, the portion of the first sidewall spacerremoved extends less than 90% of a height of the first sidewall spacer.

According to some embodiments, the first contact overlies between 1% and30% of a width of the first sidewall spacer.

The foregoing outlines features of several embodiments so that those ofordinary skill in the art may better understand various aspects of thepresent disclosure. Those of ordinary skill in the art should appreciatethat they may readily use the present disclosure as a basis fordesigning or modifying other processes and structures for carrying outthe same purposes and/or achieving the same advantages of variousembodiments introduced herein. Those of ordinary skill in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the present disclosure, and that they may makevarious changes, substitutions, and alterations herein without departingfrom the spirit and scope of the present disclosure.

Although the subject matter has been described in language specific tostructural features or methodological acts, it is to be understood thatthe subject matter of the appended claims is not necessarily limited tothe specific features or acts described above. Rather, the specificfeatures and acts described above are disclosed as example forms ofimplementing at least some of the claims.

Various operations of embodiments are provided herein. The order inwhich some or all of the operations are described should not beconstrued to imply that these operations are necessarily orderdependent. Alternative ordering will be appreciated having the benefitof this description. Further, it will be understood that not alloperations are necessarily present in each embodiment provided herein.Also, it will be understood that not all operations are necessary insome embodiments.

It will be appreciated that layers, features, elements, etc. depictedherein are illustrated with particular dimensions relative to oneanother, such as structural dimensions or orientations, for example, forpurposes of simplicity and ease of understanding and that actualdimensions of the same differ substantially from that illustratedherein, in some embodiments. Additionally, a variety of techniques existfor forming the layers, regions, features, elements, etc. mentionedherein, such as at least one of etching techniques, planarizationtechniques, implanting techniques, doping techniques, spin-ontechniques, sputtering techniques, growth techniques, or depositiontechniques such as chemical vapor deposition (CVD), for example.

Moreover, “exemplary” is used herein to mean serving as an example,instance, illustration, etc., and not necessarily as advantageous. Asused in this application, “or” is intended to mean an inclusive “or”rather than an exclusive “or”. In addition, “a” and “an” as used in thisapplication and the appended claims are generally be construed to mean“one or more” unless specified otherwise or clear from context to bedirected to a singular form. Also, at least one of A and B and/or thelike generally means A or B or both A and B. Furthermore, to the extentthat “includes”, “having”, “has”, “with”, or variants thereof are used,such terms are intended to be inclusive in a manner similar to the term“comprising”. Also, unless specified otherwise, “first,” “second,” orthe like are not intended to imply a temporal aspect, a spatial aspect,an ordering, etc. Rather, such terms are merely used as identifiers,names, etc. for features, elements, items, etc. For example, a firstelement and a second element generally correspond to element A andelement B or two different or two identical elements or the sameelement.

Also, although the disclosure has been shown and described with respectto one or more implementations, equivalent alterations and modificationswill occur to others of ordinary skill in the art based upon a readingand understanding of this specification and the annexed drawings. Thedisclosure comprises all such modifications and alterations and islimited only by the scope of the following claims. In particular regardto the various functions performed by the above described components(e.g., elements, resources, etc.), the terms used to describe suchcomponents are intended to correspond, unless otherwise indicated, toany component which performs the specified function of the describedcomponent (e.g., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure. In addition, while aparticular feature of the disclosure may have been disclosed withrespect to only one of several implementations, such feature may becombined with one or more other features of the other implementations asmay be desired and advantageous for any given or particular application.

What is claimed is:
 1. An image sensor, comprising: a photosensitivesensor; a floating diffusion node; a reset transistor comprising a firstsource/drain coupled to the floating diffusion node and a secondsource/drain coupled to a first voltage source; a source followertransistor comprising a gate coupled to the floating diffusion node anda first source/drain coupled to the second source/drain of the resettransistor; and a first elongated contact contacting the secondsource/drain of the reset transistor and the first source/drain of thesource follower transistor, wherein: the first elongated contact has afirst dimension in a horizontal cross-section and a second dimension inthe horizontal cross-section, the second dimension is perpendicular tothe first dimension, and the second dimension is less than the firstdimension.
 2. The image sensor of claim 1, comprising: a transfertransistor comprising a first source/drain coupled to the photosensitivesensor and a second source/drain coupled to the floating diffusion node.3. The image sensor of claim 1, comprising: a second elongated contactcontacting the first source/drain of the reset transistor and thefloating diffusion node, wherein: the second elongated contact has athird dimension in the horizontal cross-section and a fourth dimensionin the horizontal cross-section, the third dimension is perpendicular tothe fourth dimension, and the fourth dimension is less than the thirddimension.
 4. The image sensor of claim 1, comprising: a first sidewallspacer adjacent a gate of the reset transistor; and a second sidewallspacer adjacent the gate of the source follower transistor, wherein thefirst elongated contact overlies at least one of the first sidewallspacer or the second sidewall spacer.
 5. The image sensor of claim 4,wherein the first elongated contact contacts the at least one of thefirst sidewall spacer or the second sidewall spacer.
 6. The image sensorof claim 4, wherein the first elongated contact overlies the firstsidewall spacer and the second sidewall spacer.
 7. The image sensor ofclaim 1, wherein a first sidewall spacer of the gate of the sourcefollower transistor has a first profile in vertical cross-section and asecond sidewall spacer of the gate of the source follower transistor hasa second profile in vertical cross-section different than the firstprofile.
 8. The image sensor of claim 1, comprising: a row selecttransistor comprising a first source/drain coupled to a secondsource/drain of the source follower transistor.
 9. The image sensor ofclaim 8, comprising: a second elongated contact contacting a secondsource/drain of the row select transistor, wherein: the second elongatedcontact has a third dimension in the horizontal cross-section and afourth dimension in the horizontal cross-section, the third dimension isperpendicular to the fourth dimension, and the fourth dimension is lessthan the third dimension.
 10. The image sensor of claim 9, comprising: athird elongated contact contacting the first source/drain of the resettransistor and the floating diffusion node, wherein: the third elongatedcontact has a fifth dimension in the horizontal cross-section and asixth dimension in the horizontal cross-section, the fifth dimension isperpendicular to the sixth dimension, and the sixth dimension is lessthan the fifth dimension.
 11. An image sensor, comprising: aphotosensitive sensor; a floating diffusion node; a source followertransistor comprising a gate coupled to the floating diffusion node; anda first elongated contact contacting a first source/drain of the sourcefollower transistor, wherein: the first elongated contact has a firstdimension in a horizontal cross-section and a second dimension in thehorizontal cross-section, the second dimension is perpendicular to thefirst dimension, and the second dimension is less than the firstdimension.
 12. The image sensor of claim 11, comprising: a row selecttransistor comprising a first source/drain coupled to a secondsource/drain of the source follower transistor.
 13. The image sensor ofclaim 12, comprising: a second elongated contact contacting a secondsource/drain of the row select transistor, wherein: the second elongatedcontact has a third dimension in the horizontal cross-section and afourth dimension in the horizontal cross-section, the third dimension isperpendicular to the fourth dimension, and the fourth dimension is lessthan the third dimension.
 14. The image sensor of claim 12, wherein asecond source/drain of the row select transistor corresponds to anoutput terminal of a readout circuit of the image sensor.
 15. The imagesensor of claim 11, comprising: a transfer transistor comprising a firstsource/drain coupled to the photosensitive sensor and a secondsource/drain coupled to the floating diffusion node.
 16. An imagesensor, comprising: a photosensitive sensor; a floating diffusion node;a reset transistor comprising a first source/drain coupled to thefloating diffusion node; a source follower transistor comprising a gatecoupled to the floating diffusion node; and a first elongated contactcontacting the first source/drain of the reset transistor and the gateof the source follower transistor, wherein: the first elongated contacthas a first dimension in a horizontal cross-section and a seconddimension in the horizontal cross-section, the second dimension isperpendicular to the first dimension, and the second dimension is lessthan the first dimension.
 17. The image sensor of claim 16, comprising:a row select transistor comprising a first source/drain coupled to afirst source/drain of the source follower transistor.
 18. The imagesensor of claim 17, comprising: a second elongated contact contacting asecond source/drain of the row select transistor, wherein: the secondelongated contact has a third dimension in the horizontal cross-sectionand a fourth dimension in the horizontal cross-section, the thirddimension is perpendicular to the fourth dimension, and the fourthdimension is less than the third dimension.
 19. The image sensor ofclaim 16, wherein a first sidewall spacer of the gate of the sourcefollower transistor has a first profile in vertical cross-section and asecond sidewall spacer of the gate of the source follower transistor hasa second profile in vertical cross-section different than the firstprofile.
 20. The image sensor of claim 16, wherein: a first sidewallspacer of the gate of the source follower transistor has a surfacehaving a first portion having a first slope, a second portion having asecond slope, and a third portion having a third slope, the secondportion is between the first portion and the third portion, and thesecond slope is less than the first slope and less than the third slope.